1. Field of the Invention
This invention relates to the separation of tetramethyldiphenyl ether isomers and more particularly concerns the separation of 3,3',4,4'-tetramethyldiphenyl ether from a solid mixture thereof with 2,3,3',4'-tetramethyldiphenyl ether.
2. Description of the Prior Art
Over approximately the past two decades, organic compounds in which two or more carboxylic acid groups are bonded to one or more carboxylic and/or heterocyclic aromatic nuclei have become of increasing interest, either as direct components in or as intermediates for synthetic condensation polymer molecules. Some of the polymers containing repeating units derived from aromatic polycarboxylic acids have found broad spectrum utility in synthetic fibers and films, as well as in various types of resin formulations, whereas others of such polymers have been more limited in scope of application, but are not less useful. The more common nuclei of aromatic polycarboxylic acids that form polymers that are useful for these purposes include certain simple and more complex bridged aromatic ring systems. One important member of the group involving bridged aromatic ring systems can be pictured most simply by the formula: Ar--O--Ar.sup.1, where the groups, Ar and Ar.sup.1, represent the same or different cyclic aromatic nuclei. In some cases, the aromatic nuclei of such polycarboxylic acids will contain one or more additional ring substituents, such as amino, nitro, halogen, hydroxyl, cyano, sulfonyl, and the like groups. These additional groups do not participate in the principal polymer-forming reactions, but they can be desirable molecular constituents, either because of the properties they impart to the polymer or because they render the initial polymer molecules susceptible to modification by further reaction.
The aromatic polycarboxylic acids used in polymers generally are produced by subjecting aromatic compounds having a plurality of appropriately positioned alkyl substituents on the aromatic ring or rings to oxidation processes. Thus, the commercial importance of diphenyl ether-based monomers and diphenyl sulfide-based monomers in engineering polymer formulations is well recognized. Either such materials are themselves potentially useful monomers or can serve as precursors to monomers of proven utility such as oxybisanaline. For example, 4,4'-diaminodiphenyl ether is currently used in the preparation of Torlon.RTM. and of Vespel.RTM.. Other diphenyl ether-based monomers which are of potential commercial importance include diacids, diols or dianhydrides of diphenyl ether.
Although diphenyl ether-based monomers and diphenyl sulfide-based monomers have proven utility, their high cost and the lack of a convenient method of preparation have hindered any large volume applications of such monomers. For example, 3,3',4,4'-tetracarboxydiphenyl ether is useful as the precursor to oxybis (phthalic anhydride), a monomer used in the formulation of polyimides and can be produced by the oxidation of 3,3',4,4'tetramethyldiphenyl ether using an oxygen-containing gas and a metal-containing catalyst. One attractive method for producing 3,3',4,4'-tetramethyldiphenyl ether is the Ullmann coupling reaction between 4-bromo-o-xylene and an alkali metal phenolate of 3,4-dimethylphenol. However, because of the presence of 3-bromo-o-xylene as an impurity with 4-bromo-o-xylene, this reaction generally produces substantial amounts of 2,3,3',4'-tetramethyldiphenyl ether as a by-product, in addition to the desired 3,3',4,4'-tetramethyldiphenyl ether, and it is extremely difficult to separate these two isomers. In fact, no method for effecting this separation has been reported. Thus, it would be very desirable to develop a convenient and efficient method for separating 3,3',4,4'tetramethyldiphenyl ether from 2,3,3',4'-tetramethyldiphenyl ether.